There have heretofore been proposed a number of different types of apparatus for thermostatic control of a temperature modifying apparatus. The object of such a thermostatic control apparatus is to keep the temperature within an enclosed space at a specified point or within a specified range. It is desirable to keep the temperature within such a specified range with the minimum expenditure of energy.
The most common form of control in the prior art thermostat is on/off set point control. In this type of thermostat the temperature modifying apparatus is turned on when the measured temperature has one relation to a set point temperature and the temperature modifying apparatus is turned off when the measured temperature has another relationship to the set point temperature. In the case of control of a heating unit, the heating unit is turned on when the measured temperature is below the set point temperature and turned off when the measured temperature is above the set point temperature. Control of an air conditioning unit employs the opposite strategy.
The most common form of control in the prior art thermostat is on/off set point control. In this type of thermostat the temperature modifying apparatus is turned on when the measured temperature has one relation to a set point temperature and the temperature modifying apparatus is turned off when the measured temperature has another relationship to the set point temperature. Usually this type of control includes a dead zone. In the case of control of a heating unit, the heating unit is turned on when the measured temperature is below a first temperature and turned off when the measured temperature is above a second higher temperature. Control of an air conditioning unit employs the opposite strategy.
The first thermostats employed bimetal strips. These bimetal strips are formed of layers of two metals having differing coefficients of expansion due to changes in temperature. They thus have differing curvatures depending upon temperature. Such bimetal strips were employed to control an on/off switch based upon their curvature. The set point temperature of such a thermostat is entered manually by positioning the bimetal sensing strip.
The typical thermostat of this type included an anticipator function. When heating, for example, it is known that the ambient temperature continues to rise after the heating unit is switched off. This occurs because of the latent heat in the heating unit which has not yet been transported to the space to be controlled. Operating the heating unit until the ambient temperature exceeds a particular temperature will result in an overshoot of this temperature. A particularly advantageous manner of providing this anticipator function is a resistance heater which is switched on when the heating unit is switched on by the thermostat. Using such a heater the temperature measured by the bimetal strip rises faster than the ambient temperature resulting in the heating unit being switched off sooner than otherwise, thereby anticipating the resulting continued rise in temperature after the heating unit is switched off. A similar phenomenon occurs for cooling. To provide an anticipate function for control of cooling the resistance heater is switched on when the air conditioner is switched off by the thermostat.
This form of anticipator has an additional advantage. The resistance heater forces the thermostat to cycle the controlled apparatus at a minimum rate regardless of temperature. This serves to reduce the temperature deviations of the control function. The particular location of the thermostat determines the temperature deviations that the thermostat experiences for identical operation of the controlled apparatus. A thermostat located very near a hot air duct will experience much greater temperature deviations than a thermostat located in a closet. While the control function will result in the same average temperature in both cases, the cycle rates and the temperature deviations will differ. The thermostat located near a hot air duct will cycle frequently and will provide short periods of activation of the heating unit with short off periods. This will permit relatively low temperature deviations from the set point. The thermostat located in a closet will cycle infrequently and will provide long periods of activation followed by long off periods. This will result in large temperature deviations from the set point during the cycle. By forcing the thermostat to cycle at a minimum rate the temperature deviations are kept within reasonable limits.
This forced cycling is preferable selected to provide the proper number of cycles for the particular temperature modifying apparatus. It is known in the art that differing temperature modifying apparatuses have differing desirable operation cycles. It is generally understood in the art that hot water heating units and air conditioning units operate best when run at three cycles per hour. For hot air heating units the optimum rate is generally understood as six cycles per hour. These assumed values take into account the minimum on and off times of the particular types.
Use of such an anticipator thermostat is not ideal. It is known in the thermostat art that there is a nearly linear relationship between the duty cycle of the temperature modifying apparatus and the difference between the actual average temperature and the set point temperature for these thermostats. This phenomenon is known as droop. In the case of heating the greater the thermal load on the heating unit the greater the duty cycle, the greater the heating effect by the anticipator resistance heater and the greater the difference between the set point temperature and the actual average temperature. For example, when the exterior temperature is coldest the actual average temperature is the furthest below the set point temperature. The same type of phenomenon occurs in the opposite sense for control of air conditioning. The average temperature is most nearly the set point temperature at low duty cycles approaching 0%, thus at low thermal loads. The maximum difference between the actual average temperature and the set point in these thermostats is approximately 3.degree. F. at high duty cycles approaching 100% for these thermostats.
The typical reaction of a user to this situation leads to excess energy usage. In control of a heating unit, when the thermal load is greatest the user feels cold. At the same time the thermometer on the thermostat, which is also heated by the anticipator resistance heater, will tend to read the same as the set point. Most likely the user will raise the set point on the thermostat in order to compensate. This has the effect of raising the actual average temperature along with the set point to the range desired for comfort. This is fine for when the thermal load is high. However this has an undesirable effect when the thermal load decreases. When the thermal load is lessened the duty cycle is decreased and the difference between the set point temperature and the actual average temperature is decreased. This has the effect of maintaining a higher temperature than the minimum required for comfort, thereby unnecessarily increasing the energy usage. The user is unlikely to notice the difference and is unlikely to readjust the set point temperature when the thermal load changes. Thus excess energy usage occurs.
More recently thermostats have been constructed of electronic components. The use of a microprocessor enables more sophisticated control of temperatures. It is known in the art to enable the operator to specify a program of differing temperatures for differing times of the day, even for differing days of the week. The desired temperature or temperature range could thus be specified for greatest energy savings without sacrificing comfort. Such electronic thermostats still typically employ an on/off set point temperature control similar to the control strategy used in the bimetal strip thermostats with the desired temperature being changeable.
Typically such thermostats do not include an anticipator function but rely upon a hysteresis zone of temperatures. In the case of control of heating the thermostat will turn on the heating unit if the ambient temperature is lower than a first temperature and off if the ambient temperature is above a second higher temperature. In the zone between these two temperatures the thermostat may control the heating unit to be on or off depending upon the prior history. This causes the ambient temperature to swing between the two temperatures repeatedly crossing the hysteresis zone first while on and then while off.
This control function is good if the size of the hysteresis zone is matched to the temperature swings observed by the thermostat is its particular location. If the thermostat is in a lively location, such as in the direct flow path of air from a duct, then the hysteresis zone should be large to provide operation cycles having a reasonable rate for the particular temperature modifying apparatus controlled. Likewise a thermostat in an unlively location, such as in a closet, should be small because this location experiences small temperature swings. Typically such electronic thermostats do not provide an adjustment for the size of the hysteresis zone. In addition, even if such an adjustment were provided, it would be very difficult even for a knowledgable user to determine the proper adjustment. Such an adjustment would be beyond the understanding and capability of most users. Therefore such electronic thermostats provide a fixed hysteresis zone.
It would therefore be advantageous to provide a new type of thermostatic control strategy that would enable a better correlation between the desired temperature and actual average temperature over a wide range of thermal loads and provide a reasonable duty cycle for the temperature modifying apparatus regardless of the liveliness of the location or the thermostat.